This invention relates to imaging systems and more particularly to improved electrophotographic developer materials.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrostatographic process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic material referred to in the art as "toner." The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This powder image may then be transferred to a support surface such as paper. The transferred image may be subsequently permanently affixed to a support surface as by heat. Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a light and shadow image, one may form the latent image directly by charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvents or overcoating treatment may be substituted.
Similar methods are known for applying the electroscopic particles to the electrostatic latent image to be developed. Included within this group are the "cascade" development techniques disclosed by E. N. Wise in U.S. Pat. No. 2,618,552; the "powder cloud" technique disclosed by C. F. Carlson in U.S. Pat. No. 2,221,776; and the "magnetic brush" process disclosed, for example, in U.S. Pat. No. 2,874,063.
Development of an electrostatic latent image may also be achieved with liquid rather than dry developer materials. In conventional liquid development, more commonly referred to as electrophoretic development, an insulating liquid vehicle having finely divided solid material dispersed therein contacts the imaging surface in both charged and uncharged areas. Under the influence of the electric field associated with the charged image pattern, the suspended particles migrate toward the charged portions of the imaging surface separating out of the insulating liquid. This electrophoretic migration of charged particles results in the deposition of the charged particles on the imaging surface in image configuration. Electrophoretic development of an electrostatic latent image may, for example, be obtained by flowing the liquid developer over the image bearing surface, by immersing the imaging surface in a pool of the developer or by presenting the liquid developer on a smooth surface roller and moving the roller against the imaging surface.
Thus, in an electrophoretic development system, the entire imaging surface is contacted with the liquid developer with the charged particles separating from the carrier liquid and migrating to the charged field or image portions. The particles strongly adhere to the imaging surface by means of vanderWaals forces since the particles frequently come within about 500 angstroms of the imaging surface. Consequently, electrophoretic liquid development is particularly suitable for reproduction of continuous-tone images to produce images of high quality. Further, electrophoretic development methods may provide excellent results when applying various liquid developers of different colors successively to an electrophotographic photosensitive layer comprised of, for example, photoconductive zinc oxide which is subsequently developed to produce a multi-colored image.
In general, liquid developer compositions are prepared by suspending finely divided particles having charges of the appropriate polarity and magnitude in a highly dielectric carrier solution. To obtain excellent electrophotographic characteristics, it is generally required that the magnitude of the charges on the suspended particles does not vary with respect to the carrier solution or that the polarity of the charges is not reversed even during a prolonged storage period. Therefore, when finely divided particles in a liquid developer composition have positive charges and the developer composition is applied to an electrostatic latent image bearing negative charges, positive development results, and when applying such a composition to a positively charged latent image, reversal development results.
When finely divided toner particles made of a composition which is insoluble in a carrier solution are dispersed in the carrier liquid, these particles generally have some potential against the carrier liquid. The polarity and the magnitude of charges are usually determined depending upon the conditions of the particle surface, the molecules absorbed onto the particle surface, and the like. When particles are dispersed in a solvent such as a hydrocarbon, their polarity is usually specific to themselves. For example, pigments such as Phthalocyanine Blue, Brilliant Carmine 6B, Hansa Yellow, natural and synthetic resins such as rosin, gelatin, albumin, casein, cellulose acetate, polyvinyl acetate, polyamides, nylon, and polymethylmethacrylate have positive charges. On the other hand, pigments such as sulphur, selenium, Phthalocyanine Green, lead chromate, nitrocellulose, copolymers of vinyl chloride, polyvinylidene chloride, polyvinylidene fluoride, chlorinated polypropylene, and phenol-modified alkyd resins provide negative charges on contact with solvents of hydrocarbon systems. It has been found experimentally that the polarities of these charges are almost coincidental with the materials in a triboelectric series.
It is preferable when preparing liquid developer compositions having positive polarities for development of negative electrostatic latent images borne on a zinc oxide photosensitive layer to employ pigments or resin materials which provide positive polarities as described above. For instance, when preparing a carmine developer, it is preferable to disperse Brilliant Carmine 6B into kerosene. However, in liquid developer compositions prepared by suspending and dispersing only a pigment, the charges of the particles are often unstable under storage conditions for a prolonged period. Moreover, when developing a multi-colored image with developer compositions having different colors, it is very difficult to provide developer compositions prepared by suspending only pigments and to provide equal magnitudes of charges and to the suspended particles. Consequently, a charge adjusting agent is usually added to these compositions.
The charge adjusting agent is usually first added to the pigment and mixed sufficiently therewith and the resulting mixture is dispersed in a carrier liquid. When the charge adjusting agent is soluble in the carrier liquid, the mixture is rather easily dispersed. However, a liquid developer composition prepared as described above usually has poor storage properties, and the charges of the dispersed particles are apt to change with time. The result will be that the charge adjusting agent absorbed on the surface of the particles may be gradually released into the carrier liquid. This defect may be eliminated by using a charge adjusting agent which is insoluble in the carrier liquid. That is, the charge adjusting agent will remain on the surface of the pigment, thus the dispersed particles will retain a constant charge as against the carrier liquid since they are always combined with the charge adjusting agent. When employing a charge adjusting agent which is insoluble in a carrier solvent, it is rather difficult, however, to prepare dispersed particles which are sufficiently finely divided.
As a charge adjusting agent which is insoluble in a carrier liquid and provides a positive polarity, ethylcellulose has been so employed. However, even though ethylcellulose provides somewhat stable positive charges in a carrier liquid, there are certain disadvantages in employing this material. That is, it is sparingly soluble in solvents such as aromatic or chlorinated hydrocarbons and even when it dissolves, it frequently results in high viscosities. Thus, the viscosity of a paste or mixture comprising such a charge adjusting agent and pigment particles is increased to result in difficulty in dispersing the paste into a carrier liquid.
As a method of preparing liquid developer compositions employing a charge adjusting agent, there is one wherein a mixture, for example, composed of a pigment and a charge adjusting agent, generally a resin, is kneaded in a carrier liquid and subsequently pulverized. In such a method, however, dispersed particles of a fine particle size are rarely produced since in the pulverizing process the pigment is exposed and apt to contact directly with the carrier liquid. There is also another method wherein particles of a small particle size in a mixture consisting of a pigment and a charge adjusting agent may be prepared by spray drying and then dispersed in a carrier liquid. However, when employing this method, it is difficult to make the particles themselves and it is also difficult to redisperse the primary particles in the carrier liquid since they tend to mass together and coagulate.
It is, therefore, clear that there is a continuing need for an improved liquid developer composition.